Field of the Invention
The present invention relates to an optical element used for a camera, a video, and the like and a method for manufacturing an optical element.
Description of the Related Art
Heretofore, in a refractive optical system using the refraction of light, lenses formed from glass materials having different dispersion characteristics are used together in combination to reduce the chromatic aberration. For example, in the case of an object lens of a telescope or the like, a glass material having low dispersion and a glass material having high dispersion are used in combination as a positive lens and a negative lens, respectively, to correct the chromatic aberration which appears on the axis. However, in the case in which the configuration of lenses and the number thereof are limited and/or the case in which glass materials to be used are limited, it has been difficult to sufficiently correct the chromatic aberration in some cases.
A. D. Kathman and S. K. Pitalo have disclosed a technique in “Binary Optics in Lens Design”, International Lens Design Conference 1990, SPIE Vol. 1354, pp. 297 to 309 (hereinafter referred to as “Non-Patent Document 1”) in which a refractive optical element having a refractive surface and a diffractive optical element having a diffraction grating are used in combination to suppress the chromatic aberration with a smaller number of lenses. The technique thus disclosed utilizes a physical phenomenon that the chromatic aberration to light having a certain reference wavelength occurs in the opposite direction between a refractive surface and a diffractive surface of optical elements. In addition, by changing the period of the diffraction grating, which is continuously formed in a diffractive optical element, the same characteristic as that of an aspherical lens can be obtained.
However, one light ray incident on a diffractive optical element is divided into plural light rays of different orders by a diffraction function. At this stage, diffracted light rays of orders other than a designed order are focused on locations different from that of the light ray of the designed order, thereby causing the generation of flare.
According to Japanese Patent Laid-Open No. 2008-203821 (hereinafter referred to as “Patent Document 1”), by using an optical material having relatively low refractive index and high dispersion and an optical material having relatively high refractive index and low dispersion, a light flux in a use wavelength region is concentrated in a specific order (hereinafter referred to as “designed order”), and the intensities of diffracted light rays of diffraction orders other than the designed order are suppressed, so that the generation of flare is prevented.
In Patent Document 1, as the optical material having relatively low refractive index and high dispersion, a material having a low refractive index nd (1.48<nd<1.57), high refractive index dispersion represented by Abbe's number νd (14<νd<28), and a low second order dispersion characteristic θgF (0.34<θgF<0.47) is used. In addition, as the optical material having relatively high refractive index and low dispersion, a material having a high refractive index (1.54<nd<1.63) and low refractive index dispersion (44<νd<57) is used. According to Patent Document 1, when the optical material having relatively low refractive index and high dispersion and the optical material having relatively high refractive index and low dispersion are formed into shapes by ultraviolet curing or the like, diffraction efficiency can be improved in a wide wavelength range over the whole visible region.
The optical material having low refractive index and high dispersion of Patent Document 1 can be obtained in such a way that after a transparent conductive metal oxide in the form of fine particles which has high refractive index dispersion and a low second order dispersion characteristic is mixed with and dispersed in a binder resin having a low refractive index to form a composite material, this composite material is cured by ultraviolet light. In addition, as the transparent conductive metal oxide, transparent conductive metal oxides, such as indium tin oxide (ITO), have been disclosed.
An optical element, such as a multilayer diffractive optical element, which partially uses a dispersion material containing fine particles of a transparent conductive material, such as ITO, shows a high diffraction efficiency state over the whole visible region. However, in use environment, the optical property, that is, the refractive index, of the dispersion material containing fine particles of a transparent conductive material is changed. By the change in the refractive index, the optical properties of the optical element are degraded. For example, in the case of a multilayer diffractive optical element, high diffraction efficiency over the whole visible region is decreased.